Method of recovering oil from substantially level formation strata

ABSTRACT

A method for primary or secondary recovery of petroleum fluids from substantially level strata, and especially in formations where poor bottom hole pressure conditions are encountered, comprises providing a central cased well extending into a permeable stratum lying between two impermeable strata, providing a plurality of cased wells spaced at substantial distances from and extending in a ring or other configuration about the central well, providing communication between each of the plurality of wells and a respective permeable stratum above the first stratum, lowering the pressure in the central well to produce a flow of gaseous or liquid fluids toward the central well, utilizing high temperature steam and injection nozzles in said plurality of wells to aid the flow of fluid through the plurality of wells and downwardly from each respective stratum and into the lower stratum, and producing the fluids through the central well.

United States Patent [191 Riehl July 31,1973

METHOD OF RECOVERING 01L FROM Inventor:

Filed:

Frederick W. Riehl, 2347 Albion St.,

Denver, Colo. 80207 Mar. 1, 1972 Appl. No.: 230,577

References Cited UNITED STATES PATENTS Young. 166/314 166/268 X 166/314 X 166/272 X Spearow Brown et al.

Hippel Riehl Mesflman etal. ..166/258 Primary Examiner-Stephen J. Novosad Attorney-Wm. Griffith Edwards [57] ABSTRACT A method for primary or secondary recovery of petroleum fluids from substantially level strata, and especially in formations where poor bottom hole pressure conditions are encountered, comprises providing a central cased well extending into a permeable stratum lying between two impermeable strata, providing a plurality of cased wells spaced at substantial distances from and extending in a ring or other configuration about the central well, providing communication between each of the plurality of wells and a respective permeable stratum above the first stratum, lowering the pressure in the central well to produce a flow of gaseous or liquid fluids toward the central well, utilizing high temperature steam and injection nozzles in said plurality of wells to aid the flow of fluid through the plurality of wells and downwardly from each respective stratum and into the lower stratum, and producing the fluids through the central well.

12 Claims, 4 Drawing Figures METHOD OF RECOVEMNG OlllL FROM SUBSTANTXALLY LEVEL FORMATION STRATA This invention relates to an oil recovery method and particularly to an improved method for producing petroleum fluids from formation strata which lie substantially level.

Various methods have been provided heretofore for producing oil from formations the strata of which lie substantially level. The rate of production of petroleum fluids depends on many factors including the permeability of the reservoir formations, the existing fluid pressures, and the viscosity of the liquids in the formations. It is desirable to provide a method which effectively facilitates the production of oil from level formations and from irregular area formations and formations having poor bottom hole pressure conditions. Accordingly, it is an object of the present invention to provide a method for producing oil from formations having a plurality of substantially level strata and including an improved procedure for facilitating the removal of the oil from such formations.

It is another object of this invention to provide an improved method for effecting a flow of petroleum fluids from throughout a level stratified formation to a central producing zone.

It is another object of this invention to provide an improved method for effecting the production of petroleum fluids from large area stratified formations by utilizing a minimum number of production pumps.

it is another object of this invention to provide an improved method for utilizing steam to facilitate the production of petroleum fluids from a formation comprising a plurality of substantially level oil bearing strata.

it is a further object of this invention to provide an improved method for the effective production of petroleum fluids from substantially level stratified formations which are of irregular area configuration.

It is a still further object of this invention to provide an improved method for facilitating the production of petroleum fluids from substantially level stratified formations having adverse bottom hole pressure conditions.

Briefly, in carrying out the objects of this invention in one example hereof, a large sized production well is provided in the formation in a generally central location and a ring or other configuration of separate injection wells is provided at a substantial distance from the central well in surrounding relationship therewith and with the separate" wells spaced substantially evenly from one another. Geological data from the central well is utilized as information in regard to permeable sands, oil quality, and strata location and is used for the further work in the formation. The central well is made to extend into a permeable stratum from which produced fluids are to be withdrawn and the injection wells are arranged to extend through oil bearing formations and into the same permeable stratum as the central well. Each of the injection wells in the ring is arranged to communicate with a respective selected permeable stratum of the formation and steam may be injected through nozzles adjacent the selected stratum and which are used to produce a venturi effect and induce a flow of petroleum fluids from the formation both laterally or horizontally through the formation to the injection well and downwardly toward the producing zone. The pressure in the central well is reduced by a high capacity vacuum pump as needed and fluids are allowed to accumulate in the bottom formation which acts as a large reservoir, and, upon sufficient accumulation, the liquids are produced by a suitable large production pump through a production tubing, which extends through the central well down to the production pump. The steam employed in this method serves to reduce the viscosity of the petroleum liquids it encounters and further provides pressure differentials between the upper oil bearing strata and the lower producing zone, and during the continuation of the process the petroleum fluids from all strata are moved horizontally toward the injection wells and thence downwardly to the producing zone and are produced therefrom and transported to ground level.

The features of novelty which characterize this invention are pointed out with particularity in the claims annexed to and forming a part of this specification. The invention itself, however, together with further objects and advantages thereof will best be understood upon reference to the following description taken in connec tion with the accompanying drawings in which:

FIG. 1 is a plan view of an oil field or reservoir formation having wells located for the practice of the method of this invention;

FIG. 2 is a sectional view taken generally along the line 2-2 of FIG. 1;

FIG. 3 is an enlarged sectional elevation view of a venturi nozzle for use with the method of this invention; and

FIG. 4 is a view similar to FIG. 3 illustrating a modified arrangement of the venturi nozzle.

Referring now to the drawing, the stratified oil reservoir formation illustrated in FIGS. 1 and 2 is of a type wherein the strata are substantially level and permeable and impermeable strata occur alternately throughout the formation. A cased well 10 is provided in the central zone of the formation from which petroleum fluids are to be produced. This well may be provided by utilizing a cased well already in existence, or a well may be drilled and cased for the use of the method of this invention. As illustrated, the casing is perforated at 11 so that it is in communication with a bottom permeable stratum l2 and extends somewhat into an impermeable stratum 13 immediately below the stratum 12. The well is extended into the stratum 13 a short distance to provide a sump for location of the production pump illustrated at 14, this pump being in communication with the surface of the ground through a production tubing 15. The pump 14 may be of any suitable type and has, for purposes of illustration, been shown as an electric motor pump unit, suitable electric leads (not shown) being provided for supplying the current for driving the pump. The casing for the well 10 is arranged in comm unication with a suitable high capacity motor driven vacuum pump 16 through a top connection 17 secured to the flanged top of the casing by a flange 18. The production tubing 15 extends through the elbow of the connection 17 and is provided with a manual valve 20. If desirable, particularly in relatively deep wells, a second pump 14 may be employed as a booster for assisting the pump 14 in moving the oil to the surface. The booster pump, as shown, may be positioned in the production tube about one-third to one-half of the distance from the bottom. When the pumps are electric motor driven units the required electric supply lines weigh substantially less than the lines for a single pump at the bottom and the tandem pump arrangement is economically preferable.

The permeable formation 12 may be a petroleum fluid containing formation or a permeable sand or similar structure without oil or other liquids present. When liquids are present their production from the stratum 12 provides the voids required for the flow of liquids from the higher stratum, as described below. The well passes through a plurality of oil bearing strata above the stratum l2, and for purposes of illustration has been indicated as comprising three such strata 21, 22 and 23, respectively. In order to utilize the well 10 and pump 14 for producing oil from the strata 21, 22 and 23, a multiplicity of injection wells are drilled into the formation in a generally ring-like configuration or other encircling or surrounding configuration substantially spaced from the well 10 and extending about the outer zones of the formation to be produced. The configuration of the surrounding ring of wells may vary widely depending upon the geological characteristics or other requirements of the formation and may, for example, be rectangular or otherwise include straight line portions. For purposes of illustration, the multiplicity of wells have been indicated as drilled in groups of wells, each group comprising three wells 24, 25 and 26, as illustrated in FIG. 2. All of these wells are drilled to terminate in the stratum 12. The casings of the wells 24 are perforated as indicated at 27 to communicate with the stratum 21; the casings of wells 25 are perforated at 28 to communicate with the stratum 22; and the casings of wells 26 are perforated at 29 to communicate with the stratum 23. Thus, three encircling lines of wells are provided, each communicating with a different stratum and containing respectively the wells 24, 25 or 26; the wells in each set being substantially evenly spaced. The spacing between the wells in different sets and which communicate with different strata need not be the same as illustrated and may be selected for convenience. These injection wells provide paths for the flow of oil downwardly to the formation 12.

The casings of each of the wells 24, 2S and 26 are provided with manual valves at the surface, as indicated at 31, 32 and 33, respectively, and are connected to receive steam at high pressure and temperature through steam lines 34, 35 and 36, respectively. The steam lines are connected to a suitable boiler or boilers for supplying the steam under the temperature and pressure conditions required for the production operation.

Each of the wells 24, 25 and 26 is provided with ajet or nozzle 37, 38 and 39, respectively, arranged adjacent the perforations in the respective well at 2'7, 28 and 29 and each nozzle is arranged to utilize a jet of high pressure steam to produce a low pressure zone adjacent the perforations and to induce a flow of fluid horizontally through the respective stratum toward the respective wells 24, 25 and 26 and then downwardly into the lower portion of the respective well casing and thence to the stratum 12.

FIG. 3 illustrates one form of injection nozzle suitable for use as the nozzles 37, 3B and 39. In FIG. 3, the nozzle illustrated has been indicated by the numeral 37 and the cased well as 24. As illustrated, the casing is provided with a plurality of openings 42 formed by perforating the casing wall above the nozzle. The openings 42 permit steam passing down through the casing to flow out into the formation and heat oil in the formation to lower the viscosity of the oil and increase the rate of flow horizontally through the formation and then downwardly through the well casing. The circulation of steam into the formation through the perforations 42 and then back through the perforations 27 thus facilitates the movement of oil from the formation into the well casing. The nozzle 37 is positioned toward the lower end of the fluid-bearing formation 21 where the casing is provided with the perforations 27 laterally of the nozzle. Thus, the perforations 27 are available for affording a flow of oil or other fluids horizontally through the formation and into the interior of the casing upon creation of the low pressure at the nozzle discharge. All of the well casings are cemented in. As indi cated in FIG. 2, the cementing of the casing 24 to the impermeable formations above and below the perforation formation 21 is indicated at 44 and 45, respectively.

For some applications, particularly those involving relatively shallow producing formations, it may be desirable to provide an independently movable steam line and nozzle. Thus, as indicated in FIG. 4, a separate steam line may be inserted in the injection well casing and moved with precision to the optimum level with respect to a permeable stratum thereby producing the low pressure at the most desirable position with respect to the stratum. In FIG. 4, a separate steam line 50 is moved down the casing of the well 24, by way of example, until a nozzle 51 at the bottom end of the line is in position just below the perforations indicated at 52 in the well casing. The use of a separate steam line facilitates the locating of the venturi nozzle precisely with respect to the permeable stratum and further allows the line to be withdrawn should it be desired to introduce a large volume of steam for heating of the formation and lowering of the viscosity of the liquid therein.

The flow of steam through the nozzles in the wells 24, 25 and 26 may be employed to increase the natural flow of petroleum fluids, but for many applications these nozzles will not be employed until the natural flow downwardly through the injection wells has been reduced to a very low flow or has ceased to exist.

During the production of the reservoir formation when all of the wells have been provided as indicated, the vacuum pump 16 is operated to produce a low pressure within the casing of the well 10 thereby inducing a flow of fluids, both liquid and gaseous, through the stratum 12 toward the well 10. The natural pressures in the several permeable strata may be such that there is, at least at the beginning of the production period, a substantial flow of petroleum fluid from the strata to the respective wells 24, 25 and 26, downwardly into the stratum 12. It will be understood in this connection that the natural pressures in the various strata may be significantly different such that there may be an initial tendency toward a flow from one of the strata into one or more of the strata 21, 22 or 23. Such flow tendency may be overcome by the admission of steam through the respective nozzles 37, 38 and 39, to produce a lower pressure adjacent the particular strata and induce a flow of fluids from that strata into the stratum 12. Furthermore, the viscosity of petroleum liquids in the several strata may be different and in the event that the viscosity is high the steam may be utilized to heat the liquid and lower its viscosity and thereby increase its ability to flow horizontally into the wells and thence to the stratum l2.

Depending upon the pressure and viscosity conditions in the several strata of the formation, steam is admitted to the wells in the peripheral ring, as required, and all of the wells may thus be brought to production and to discharge produced fluids downwardly into the stratum 12 from which the liquids are withdrawn by operation of the pump 14; gaseous fluid is removed through the casing by operation of the pump 16. Suitable sensing devices (not shown) may be provided to monitor the condition of the fluids flowing from the several strata and may be utilized to control the pumps 14 and 16 in a manner to effect maximum production of fluids from the formation.

The condition of the fluids in the stratum 12, if fluids are present, is also monitored and steam admitted to the formation 12 through the wells 24, and 26 is effective to reduce the viscosity of the liquids in this formation and facilitate the production of this liquid as well as that flowing from the upper strata.

The locations of the central well 10 and the outer wells 24, 25 and 26 are determined in the light of knowledge of the geological characteristics of the formation and the spacings between the wells 24, 25 and 26 and the central well, as well as the spacings between the wells 24, 25 and 26 are also selected to provide optimum location of the wells for the practice of the foregoing method of production. The total area of oil field in which the method of this invention may be employed will depend upon the characteristics of the field, and for some fields additional surrounding wells also in ring-like formations may be provided, as indicated in FIG. 1 by the numerals 24', 25 and 26', indicating groups of three wells arranged in a second configuration about the first configuration including the wells 24, 25 and 26 in groups of three. The second ring of wells may be spaced from the central well twice as far as the first ring, and other additional rings may be employed in a similar manner. In the application of the method of this invention, the bottom formation, designated by the numeral 12, may or may not include liquids, and if petroleum liquids are present in this formation they are produced first to an extent sufficient to provide voids in the formation into which the fluids produced from the upper formations 211, 22 and 23 may flow. Single large production pump M and vacuum pump 16 may be employed so that the entire production of petroleum may take place from a central well, as indicated in the drawings. In one configuration of the central formation it may be found desirable to provide two or more pro duction pumps surrounded by injection wells in the manner illustrated. The injection wells 24, 25 and 26 are arranged in groups of three, each serving a respective one of the strata 21, 22 and 23 so that the distribution of injection wells for any one stratum is at substantially uniform spacing. It is contemplated that for the most part production will be effected by natural pressures so long as these pressures prevail and that the method of this invention will be applied in most applications after the natural flow has ceased.

As the upper formations become depleted toward the end of the project, the liquids flowing through the bottom permeable stratum toward the pump M will gradually leave voids and the steam under pressure will approach the production pump and thereby maintain a pressure difference to facilitate the production of the final producible liquid in the formation.

By way of example, and not by way of limitation, in an oil field of the order of 250 to 400 acres in size and having a central well provided as described above, the steam injection wells 24, 2S and 26 may be located at an average distance of say 2,000 feet from the central well and adjacent injection wells may be located about 600 feet apart. Each of these wells may be provided with a separate steam line extending through the respective casing and terminating in the respective injection nozzles 37, 38 and 39, and each of the nozzles is located at or near the bottom of its respective producing formation stratum. Steam is supplied when required because of high viscosity of the reservoir liquids or because of pressure differences existing between the strata of the formation and is utilized in order to maintain a continuous downward flow of oil to the reservoir formation 12 which has been prepared by operation of the pump 16. The pump 14 is operated whenever liquids are present in sufficient quantity at the bottom of the well 10 until all of the permeable strata 21, 22 and 23 have been depleted and the formation effectively exhausted.

For many applications, the existing bottom hole pressures, together with the differentials produced by operation of the steam jets, will produce ample pressure differentials to cause the oil to flow freely and this free flow will be facilitated by the heat supplied from the steam and the consequent reduction in the viscosity of the liquid. In some oil fields, because of contour or shape of the formation, the sizes of the injection wells arranged about the center wells may be different in order to employ the injection effect most efficiently and to effect full production from all areas of the field.

Under some production conditions it may be found desirable to supply steam to less than the entire ring of injection wells which are in communication with one of the strata. Thus, one or more of the injection wells may be supplied with steam while the others are not.

l claim:

1. The method of recovering petroleum fluid from a reservoir formation having a plurality of substantially level permeable strata separated by and lying between impermeable strata which comprises:

providing a first well having a casing and extending into a first permeable stratumbelow a first impermeable stratum lying between said first permeable stratum and a second and higher permeable stratum;

providing an injection well spaced from said first well and passing through said second permeable stratum and through said first impermeable stratum into said first permeable stratum and a casing for said injection well;

providing openings through the casing of said injection well for communication with said second permeable stratum; lowering the pressure in said first well to induce movement of fluid through said first permeable stratum toward said first well and to tend to produce a flow of fluid from said second permeable stratum through said openings and into said injection well whereby fluid flows from said secondpermeable stratum into said first permeable stratum;

collecting liquid adjacent the lower end of said first well;

providing a production tubing extending through the casing of said first well; and

pumping liquid from said first permeable stratum through said production tubing to the surface. 2. The method of recovering petroleum fluid from a reservoir formation having a plurality of substantially level permeable strata separated by and lying between impermeable strata as set forth in claim 1 including the step of supplying high temperature high pressure steam to said injection well for heating fluid therein and for inducing a flow of petroleum fluid from said second permeable stratum through said well to said first permeable stratum.

3. The method of recovering petroleum fluid from a reservoir formation having a plurality of substantially level permeable strata separated by and lying between impermeable strata as set forth in claim 2 including providing a nozzle in the path of the steam supplied to said injection well and positioned near the bottom portion of said second permeable stratum for producing a low pressure zone adjacent said second permeable straturn.

4. The method of recovering petroleum fluid from a reservoir formation having a plurality of substantially level permeable strata separated by and lying between impermeable strata as set forth in claim 1 including the additional step of providing a second injection well spaced from said first well and passing through a third permeable stratum in the formation and into said first permeable stratum, and providing communication between said second injection well and said third permeable stratum whereby fluid flows from said third permeable stratum into said first permeable stratum, and fluids from said first and second injection wells are produced through said production tubing.

5. The method of recovering petroleum fluid from a reservoir formation having a plurality of substantially level permeable strata separated by and lying between impermeable strata which comprises:

providing a first well extending into a first permeable stratum below a first impermeable stratum separating said first permeable stratum from a second permeable stratum and a casing for said well;

providing a multiplicity of spaced wells arranged in a generally aligned group spaced from and surrounding said first well and through said second permeable stratum and through said first impermeable stratum into said first permeable stratum and a casing for each of said multiplicity of wells;

providing openings through the casings of said multiplicity of wells for communication with said second permeable stratum;

lowering the pressure in said first well to induce movement of fluid through said first permeable stratum toward said first well and to tend to produce a flow of fluid through said openings and through said multiplicity of wells from said second permeable stratum into said first permeable stratum;

collecting liquid adjacent the lower end of said first well;

providing a production tubing extending through the casing of said first well; and

pumping liquid from said first permeable stratum through said production tubing to the surface.

least one of said multiplicity of wells for reducing the viscosity of petroleum liquids in said formation.

7. The method of recovering petroleum fluid from a reservoir formation as set forth in claim 6 including the step of providing injector nozzles in said at least one of said multiplicity of wells adjacent said second permeable stratum for reducing the pressure in said second permeable stratum to facilitate the flow of fluids from said second permeable stratum through said at least one of said multiplicity of wells and into said first permeable stratum.

8. The method of recovering petroleum fluid from a reservoir formation as set forth in claim 5 including the step of providing a similar generally aligned group of spaced wells surrounding said first mentioned group and spaced outwardly therefrom and producing fluids from said second group through said production tubing in the same manner.

9. The method of recovering petroleum fluids from a reservoir formation having at least three substantially level permeable strata separated by and lying between impermeable strata which comprises:

providing a first well extending into a first permeable stratum below a first impermeable stratum separating said first permeable stratum from a second per meable stratum and a casing for said well;

providing a first set of spaced wells arranged in a generally aligned configuration spaced from and surrounding said first well and extending through said second permeable stratum and through said first impermeable stratum into said first permeable stratum and a casing for each of said multiplicity of wells;

providing openings through the casings of said first set of spaced wells for communication with said second permeable stratum;

providing a second set of spaced wells arranged in a generally aligned configuration spaced from and surrounding said first well and passing through said second permeable stratum and a third permeable stratum and through said first impermeable stratum into said first permeable stratum, and a casing for each of the wells of said second set;

providing openings through the casings of said wells of said second set for communication with said third permeable stratum;

lowering the pressure in said first well to induce movement of fluids through said first permeable stratum toward said first well and to tend to produce a flow of fluids through said openings in the casings of said first and second set of injection wells respectively from said second and third permeable strata into said first permeable stratum;

collecting liquid adjacent the lower end of said first well;

providing a production tubing extending through the casing of said first well; and

pumping liquid from said first permeable stratum through said production tubing to the surface.

10. The method of recovering petroleum fluid from a reservoir formation having a plurality of substantially level permeable strata separated by and lying between 5 impermeable strata which comprises:

6. The method of recovering petroleum fluid from a reservoir formation as set forth in claim 5 including the step of introducing high temperature steam through at providing a first well extending into a first permeable stratum in the lower portion of the formation to be produced and a casing for said well;

tion wells surrounding and substantially spaced;

from said first well and extending downwardly through a plurality of permeable strata of said reservoir formation and into said first permeable stratum and a casing for each of said injection wells;

said injection wells being arranged in groups, one group for each permeable strata from which fluids are to be produced;

providing openings through the casings of said injection wells for communication with respective ones of said permeable strata from which fluids are to be produced, corresponding wells in order of each of said groups being held in communication with the same respective different one of said strata whereby each of said groups includes one respective well in communication with each different one of said strata;

lowering the static pressure in said first well to induce movement of fluids through said first permeable stratum toward said first well and to tend to produce a flow of fluids through the openings in said injection well casings from each respective strata into said first permeable stratum;

collecting liquid adjacent the lower end of said first well;

providing a production tubing extending through the casing of said first well; and

pumping liquid from said first permeable stratum through said production tubing to the surface.

11. The method of recovering petroleum fluid from a reservoir formation comprising a plurality of substantially level strata, including a plurality of permeable strata each lying between impermeable strata which comprises:

providing a first well extending into a first and lower stratum of the formation; producing a vacuum and extracting vaporous fluids through said well from a central zone of said first and lower permeable stratum; providing a plurality of injection wells extending through said strata and spaced from and arranged about said central zone and entering said first stratum; providing means for allowing liquid petroleum to flow by gravity through said injection wells downwardly from an upper one of said permeable strata for accumulation in said lower stratum; passing steam under pressure through said injection wells to induce flow of liquid from said upper stratum and to heat the petroleum liquid and facilitate the downward flow of petroleum liquid through said injection wells; and producing liquid petroleum from said central zone of said first and lower stratum. 12. The method of recovering petroleum fluid from a reservoir formation as set forth in claim 1] including the step of providing first and second liquid pumps and a liquid production line,

connecting said first pump to deliver liquid to said line from said central zone; connecting said second pump in said liquid line at a distance of from about one-third to about one-half the length of said line; and operating said pumps in tandem to effect the production of liquid petroleum from said central zone. 

1. The method of recovering petroleum fluid from a reservoir formation having a plurality of substantially level permeable strata separated by and lying between impermeable strata which comprises: providing a first well having a casing and extending into a first permeable stratum below a first impermeable stratum lying between said first permeable stratum and a second and higher permeable stratum; providing an injection well spaced from said first well and passing through said second permeable stratum and through said first impermeable stratum into said first permeable stratum and a casing for said injection well; providing openings through the casing of said injection well for communication with said second permeable stratum; lowering the pressure in said first well to induce movement of fluid through said first permeable stratum toward said first well and to tend to produce a flow of fluid from said second permeable stratum through said openings and into said injection well whereby fluid flows from said second permeable stratum into said first permeable stratum; collecting liquid adjacent the lower end of said first well; providing a production tubing extending through the casing of said first well; and pumping liquid from said first permeable stratum through said production tubing to the surface.
 2. The method of recovering petroleum fluid from a reservoir formation having a plurality of substantially level permeable strata separated by and lying between impermeable strata as set forth in claim 1 including the step of supplying high temperature high pressure steam to said injection well for heating fluid therein and for inducing a flow of petroleum fluid from said second permeable stratum through said well to said first permeable stratum.
 3. The method of recovering petroleum fluid from a reservoir formation having a plurality of substantially level permeable strata separated by and lying between impermeable strata as set forth in claim 2 including providing a nozzle in the path of the steam supplied to said injection well and positioned near the bottom portion of said second permeable stratum for producing a low pressure zone adjacent said second permeable stratum.
 4. The method of recovering petroleum fluid from a reservoir formation having a plurality of substantially level permeable strata separated by and lying between impermeable strata as set forth in claim 1 including the additional step of providing a second injection well spaced from said first well and passing through a third permeable stratum in the formation and into said first permeable stratum, and providing communication between said second injection well and said third permeable stratum whereby fluid flows from said third permeable stratum into said first permeable stratum, and fluids from said first and second injection wells are produced through said production tubing.
 5. The method of recovering petroleum fluid from a reservoir formation having a plurality of substantially levEl permeable strata separated by and lying between impermeable strata which comprises: providing a first well extending into a first permeable stratum below a first impermeable stratum separating said first permeable stratum from a second permeable stratum and a casing for said well; providing a multiplicity of spaced wells arranged in a generally aligned group spaced from and surrounding said first well and through said second permeable stratum and through said first impermeable stratum into said first permeable stratum and a casing for each of said multiplicity of wells; providing openings through the casings of said multiplicity of wells for communication with said second permeable stratum; lowering the pressure in said first well to induce movement of fluid through said first permeable stratum toward said first well and to tend to produce a flow of fluid through said openings and through said multiplicity of wells from said second permeable stratum into said first permeable stratum; collecting liquid adjacent the lower end of said first well; providing a production tubing extending through the casing of said first well; and pumping liquid from said first permeable stratum through said production tubing to the surface.
 6. The method of recovering petroleum fluid from a reservoir formation as set forth in claim 5 including the step of introducing high temperature steam through at least one of said multiplicity of wells for reducing the viscosity of petroleum liquids in said formation.
 7. The method of recovering petroleum fluid from a reservoir formation as set forth in claim 6 including the step of providing injector nozzles in said at least one of said multiplicity of wells adjacent said second permeable stratum for reducing the pressure in said second permeable stratum to facilitate the flow of fluids from said second permeable stratum through said at least one of said multiplicity of wells and into said first permeable stratum.
 8. The method of recovering petroleum fluid from a reservoir formation as set forth in claim 5 including the step of providing a similar generally aligned group of spaced wells surrounding said first mentioned group and spaced outwardly therefrom and producing fluids from said second group through said production tubing in the same manner.
 9. The method of recovering petroleum fluids from a reservoir formation having at least three substantially level permeable strata separated by and lying between impermeable strata which comprises: providing a first well extending into a first permeable stratum below a first impermeable stratum separating said first permeable stratum from a second permeable stratum and a casing for said well; providing a first set of spaced wells arranged in a generally aligned configuration spaced from and surrounding said first well and extending through said second permeable stratum and through said first impermeable stratum into said first permeable stratum and a casing for each of said multiplicity of wells; providing openings through the casings of said first set of spaced wells for communication with said second permeable stratum; providing a second set of spaced wells arranged in a generally aligned configuration spaced from and surrounding said first well and passing through said second permeable stratum and a third permeable stratum and through said first impermeable stratum into said first permeable stratum, and a casing for each of the wells of said second set; providing openings through the casings of said wells of said second set for communication with said third permeable stratum; lowering the pressure in said first well to induce movement of fluids through said first permeable stratum toward said first well and to tend to produce a flow of fluids through said openings in the casings of said first and second set of injection wells respectively from said second and third permeable strata into said first permeable stratum; collecting liquid adjacent the lower end of said first well; providing a production tubing extending through the casing of said first well; and pumping liquid from said first permeable stratum through said production tubing to the surface.
 10. The method of recovering petroleum fluid from a reservoir formation having a plurality of substantially level permeable strata separated by and lying between impermeable strata which comprises: providing a first well extending into a first permeable stratum in the lower portion of the formation to be produced and a casing for said well; providing a ring-like configuration of spaced injection wells surrounding and substantially spaced from said first well and extending downwardly through a plurality of permeable strata of said reservoir formation and into said first permeable stratum and a casing for each of said injection wells; said injection wells being arranged in groups, one group for each permeable strata from which fluids are to be produced; providing openings through the casings of said injection wells for communication with respective ones of said permeable strata from which fluids are to be produced, corresponding wells in order of each of said groups being held in communication with the same respective different one of said strata whereby each of said groups includes one respective well in communication with each different one of said strata; lowering the static pressure in said first well to induce movement of fluids through said first permeable stratum toward said first well and to tend to produce a flow of fluids through the openings in said injection well casings from each respective strata into said first permeable stratum; collecting liquid adjacent the lower end of said first well; providing a production tubing extending through the casing of said first well; and pumping liquid from said first permeable stratum through said production tubing to the surface.
 11. The method of recovering petroleum fluid from a reservoir formation comprising a plurality of substantially level strata, including a plurality of permeable strata each lying between impermeable strata which comprises: providing a first well extending into a first and lower stratum of the formation; producing a vacuum and extracting vaporous fluids through said well from a central zone of said first and lower permeable stratum; providing a plurality of injection wells extending through said strata and spaced from and arranged about said central zone and entering said first stratum; providing means for allowing liquid petroleum to flow by gravity through said injection wells downwardly from an upper one of said permeable strata for accumulation in said lower stratum; passing steam under pressure through said injection wells to induce flow of liquid from said upper stratum and to heat the petroleum liquid and facilitate the downward flow of petroleum liquid through said injection wells; and producing liquid petroleum from said central zone of said first and lower stratum.
 12. The method of recovering petroleum fluid from a reservoir formation as set forth in claim 11 including the step of providing first and second liquid pumps and a liquid production line, connecting said first pump to deliver liquid to said line from said central zone; connecting said second pump in said liquid line at a distance of from about one-third to about one-half the length of said line; and operating said pumps in tandem to effect the production of liquid petroleum from said central zone. 